The alleged rivalry between Prince William and Princess Kate and the Duke and Duchess of Sussex has been well documented. A new book by a royal expert sheds more light on why issues started to arise between the two couples.

In his new book, Catherine, the Princess of Wales: A Biography of the Future Queen, royal expert Robert Jobson discussed the events that caused the ongoing royal rift. In an excerpt published by the Daily Mail, Robson wrote, “At first, public reaction to Meghan was overwhelmingly positive, with ‘the new kids on the block’ hogging most of the headlines—to the point that William and Catherine may have unconsciously raised their game. There were whispers of pettiness, even jealousy.”

Once the trouble had started, the relationship between the two royal couples started to disintegrate. “Cracks between the two couples continued to widen—with the help of petty slights, whether imagined or otherwise,” Jobson explained. Soon, relationships seemed irreparable.

While it’s unclear whether Kate and William were actually jealous of Meghan and Harry, there was an obvious shift in their interactions with one another.

Royal expert Jobson also revisited an infamous moment that allegedly took place between Kate and Meghan, and hinted at discord. “There was the time Meghan asked to borrow some lip gloss, and Harry felt Catherine had handed hers over reluctantly,” Jobson claimed. “According to him, Catherine ‘grimaced’ after Meghan had squeezed some onto a finger and applied it to her lips.”

Unfortunately, Prince Harry and Prince William, who once seemed so close, were suddenly at odds, too. “Inevitably, details leaked of the royal brothers’ troubled relationship, and the fact that Catherine and Meghan had also fallen out,” Jobson wrote.

Sadly, William and Harry’s once close brotherly bond hasn’t shown any signs of improvement in the years since. In a recent interview for ITV documentary Tabloids on Trial, Harry noted that he believes it’s still too dangerous for Meghan to return to the U.K. without adequate security.